Gwylla in terms of improvements is looking to get as many spells and rituals as possible, focusing on utility. As far as arcane tradition, diviner or transmuter is most likely, and I'm not super sold on any feats, so maybe ASIs at least at 4th and 8th level.
I was looking at the traditions and for this style of game I thought Abjuration might come in handy/be a good fit. Dispel magic and counterspell being very useful spells. And when you can project the ward - it may add in some great RP. Although any thing will fit (sep maybe necromancer don't see Gwylla going that direction).
Remember this is old school AD&D so there are plenty of scrolls to be found and a spellbook here and there. Some feats will Increase your ability and add to the character as well - Keen Mind for Gwylla seems fitting, we can even slow play it now for tidbits of information. (and the instinctual knowing which way is north is good underground).